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Cosmic Journeys: The Asteroid that Flattened Mars
Poziom:
Temat: Nauka i technologia
Just about every two years, the planet Mars
makes its closest approach to Earth... around
36 million miles.
That's when we pack our robotic emissaries
off to the Red Planet, timing their launches
to spend the least effort to get there.
Some fly around it... snapping pictures...
Others land ... to sample its surface....
...a few to crawl around its canyons and craters.
These probes may pave the way for human explorers...
and, perhaps permanent settlers... who'll
dig deeper still... in search of answers to
our most pressing question:
Did Mars develop far enough - and stay that
way long enough - for life to arise?
And, if so, does anything live now within
Mars' dusty plains... beneath its ice caps...
or maybe somewhere underground?
Mars does not give up its secrets easily
... it's almost as if the little planet is
embarrassed.
Over a century ago, a few observers thought
they saw clues that Mars is alive.
In 1877, the Italian astronomer Giovanni Schiaparelli
noted markings... which he saw as a latticework
of lines. He called them "canali" in Italian...
meaning nothing more than "shallow channels"
in English.
American astronomer, Percival Lowell, found
the lure of these features irresistible.
He saw Schiaparelli's channels as artificial
canals. He speculated that they carried melting
snow from the poles to the dry interior.
After all, on Earth, the Suez Canal had recently
opened to ship traffic. The Panama Canal was
beginning to be dug.
The Martian canals, Lowell said, were built
by a sophisticated society confronting an
environmental catastrophe on the grandest
of scales.
Those Martians, he thought, must face urgent
choice: move water across vast arid regions,
or perish on an increasingly dry planet.
As the 19th Century gave way to the 20th,
Lowell took his case to the public, in a series
of three best-selling books.
And the public responded with... questions.
Who were these Martians, who had the means
to remake an entire planet?
Some offered schemes for making contact. Giant
mirrors would flash greetings... Light beams...
Mental telepathy.
Many astronomers grew deeply skeptical...
but Lowell's vision of a harsh, yet Earth-like
planet endured in the public's imagination..
That vision was dealt harsh blow in 1964.
The Mariner Four spacecraft ventured in for
a closer look... And what it saw looked like
the Moon.
Three more Mariners followed.
They found huge dormant volcanoes... the deepest
and longest canyon in the solar system...but
not a trace of life, present or past.
In the mid-1970's, two lander-orbiter robot
teams, named Viking, took up residence at
Mars.
Maybe the Martians were just hiding, so theVikings
tested the soil for signs of life.
But all the evidence from Viking told us...
Mars is not only barren... but in fact hostile
to life.
It's no wonder. Martian air temperatures range
from -20 degrees Fahrenheit to down below
-200.
It's also very, very dry. The Sahara Desert
on Earth is a rainforest, by comparison. If
all of the water vapor in Mars' thin atmosphere
fell as snow, it would make a layer of frost
not thicker than your fingernail.
On Earth, impact craters erode over time from
wind and water... and even volcanic activity.
On Mars, they can linger for billions of years.
But so can the imprint of riverbeds, lake
bottoms and ocean shorelines... And the Viking
orbiters saw a lot of them.
It's not hard to believe that a great deal
of water once flowed here.
But where did all the water go?
To find out, scientists needed to do real
field-geology on Mars. They needed rovers...
travelling robots with tools and instruments.
In 2004, the heroic twin rovers Spirit and
Opportunity bounced down... stood up... and
drove off, ready to roam the Red Planet.
Opportunity had come to rest in a small crater
at a spot called Meridiani Planum... and it
didn't have far to travel.
For here, in plain view, on the crater wall
before it was exposed bedrock... the first
ever seen on Mars.
Not far away, Opportunity found layered rocks
on the face of a cliff. On Earth, rocks like
these typically form in sedimentary layers
at the bottom of ancient oceans.
And at every turn, Opportunity rolled across
tiny, smooth, almost perfectly round pellets...
called "blueberries" because they appeared
dark bluish-brown against Mars' rust-colored
surface.
Initially thought to be volcanic in origin,
these blueberries proved to be iron-rich spherules
of exactly the type formed within cavities
at the bottom of an ocean.
And the rovers has another trick up their
mechanical sleeves; they could drill into
the rock itself... brush it clean ... and
nuzzle a spectrometer right up to the virgin
minerals.
They found that up to 40% of the rocks were
sulfate salt... a tracer for standing water.
That wasn't the only evidence. Spirit's broken
wheel, dragging behind it, exposed soils saturated
in salt.
Clearly these rocks formed in water. But how
long ago? And, if you needed to find water
on Mars right now, where would you look?
One answer... the North Pole. From orbit,
it seemed to be covered in frozen carbon dioxide
- what we call dry ice. But might water ice
exist below the surface?
Enter Mars Phoenix... a lander that arrived
at the North Pole in early 2008.
Phoenix used rockets, not airbags, to touch
down in what appears to be the floor of an
ancient ocean.
The lander's descent thrusters blew away the
top layer of soil, and Phoenix snapped some
pictures. It sure looked like ice.
Mission managers instructed the robot to conduct
a simple experiment... reach out, dig a trench...
then watch what happens.
As expected, clumps of white stuff appeared.
A couple of days later, it was gone... vaporized.
That means it can't be salt. Or frozen carbon
dioxide, which is stable in the cold dry temperatures
of the Martian pole.
So it had to be water... the first ever directly
seen on Mars.
In fact, Phoenix probably saw droplets of
liquid salty water form and merge on its legs.
And there are other indications that the North
Pole was actually warm enough in the recent
past for water to become liquid.
Beginning in November of 2006, Mars Reconnaissance
Orbiter flew over the pole, sending out radar
pulses to peer beneath the surface.
These data have revealed that the ice cap,
just over a mile thick, formed in a succession
of layers as the climate alternated between
warm and cold.
Martian mood swings like this happen because
the planet's spin is not stabilized by a massive
moon, like that of Earth. Mars can really
wobble, with one pole tilting toward the sun
for long periods.
Whatever water's here now, is tiny compared
to what once was. There's a growing body of
evidence that early in its history, Mars was
on track to become positively Earth-like.
But something went horribly wrong.
Not long after its birth, the planet's molten
interior welled up to spawn volcanoes and
hot springs... spewing gas that formed an
early atmosphere.
As the atmosphere thickened, carbon dioxide
began to trap heat from sunlight; in other
words, the classic greenhouse effect.
Temperatures rose high enough to allow liquid
water to flow on the surface.
What then caused these waterways to evaporate...
and the planet to grow cold and dry?
In search of the answer, the Mars Global Surveyor
spacecraft was brought down to a low altitude...
and it made an intriguing detection.
MGS documented the presence of a small magnetic
field emanating from the planet.
It's incredibly weak compared to that of Earth.
Our magnetic field is generated by molten
rock deep in Earth's core that rises and falls
into a huge region below the outer curst,
called the mantle.
That has turned our planet into an electric
dynamo... the rising and sinking motion within,
combined with the spinning motion of the planet,
generates a strong magnetic field.
This field is strong enough, and extends far
enough into space, to deflect the wind of
high-energy solar particles... protecting
Earth and its atmosphere.
In its early years, Mars too appears to have
had a robust magnetic field.
Rocks in some of the older craters bear a
strong imprint of this field.... While newer
ones indicate a much weaker field.
So what caused Mars' magnetic field to drain
away... exposing the planet to solar winds?
The answer lies in its violent past... a past
that today is still written on the landscape.
This is a simple elevation map of Mars' surface,
from data gathered by the Mars Odyssey spacecraft.
The south pole, colored in orange and red,
is piled high with ice that comes and goes
with the seasons.
Moving off these southern highlands, we make
our way north.
The landscape is pocked with craters. The
largest and oldest ones have slowly faded,
softened by windblown dust.
We now pass into the Tharsis region, along
the equator. Here, on a vast high altitude
plateau, are a series of enormous volcanoes...
...including the largest one in our solar
system, Olympus Mons.
Just to the East is the great Valles Marineris.
It's named for the Mariner Nine mission that
found this vast gash in the Martian landscape.
It's about 2500 miles long and up to 400 miles
wide.
Dust devils sweep along the plains above it...
And dust blows up the valley slopes. We can
see this incredible landform in a realistic
rendering of the data.
Giant landslides have caused the walls to
slump off and pile onto the valley floor.
Feeding into this valley: a maze of side channels.
Scientists think these and other tributary
features were formed when water underground
flowed into the main basin, and the land above
collapsed.
Wider parts of the canyon are regarded as
possible landing sites for human missions.
They offer flat surfaces... and possibly access
to liquid water that may remain below the
surface even today.
From the layers of rock that line the canyon
walls... and layers of dust on the canyon
floor... future science prospectors can fill
in details of the planet's history.
The theory is that the great Valles Marineris
formed early in Mars' history, as the planet
cooled. Its huge sides were pulled apart as
the Tharsis region, just to the west, began
to rise up.
But there seems to have been another, much
larger event that changed Mars' course forever.
Travel north, down the slopes of Mars's great
volcanoes. The elevation drops as we dive
into what appears to have been an immense
ocean.
With this vast basin in the north, and the
very high elevations of the South, Mars is
a lopsided planet.
And here's why:
Early on, when the Solar System was young,
Mars was apparently hit by at least 15 large
asteroids.
One of them was large enough to turn most
of Mars' northern hemisphere into one huge
impact feature.
Earth, too, was walloped by objects this big.
But because Mars is small, only about half
the size of Earth - and has only about a third
Earth's surface area - ... the effect would
have been much more profound... and long-lasting.
The thinking is that these impacts heated
the outer subsurface layer of the planet,
the "mantle." That prevented molten rock in
its core from rising up... and switched off
Mars' magnetic field...probably forever.
Without its electromagnetic "deflector shield,"
solar radiation began to lash the planet...
gradually stripping it of its atmosphere...
and thus it's blanket of warm air. What water
Mars had would have vaporized into space...
or frozen underground.
Proof that Mars once had a thicker atmosphere
may have landed, literally, in Opportunity's
backyard.
Just recently, it spied a strange bluish rock...
a nickel iron meteorite, sitting out on the
surface.
Streaking through a thin atmosphere, this
massive a chunk of metal should have made
a much bigger dent. But a much more dense
atmosphere would have slowed its fall and
melted it in just the way that Opportunity
now sees it.
Now there are a few competing theories that
explain the disappearance of Mars magnetic
field and its atmosphere.
What's certain is that at some point early
in its history, the Red Planet grew increasingly
desolate... and that the tragedy happened
fairly quickly.
The Martian landscape we see today is replete
with coded signals from those early times...
ancient riverbeds and lake bottoms, flood
plains, and volcanic cones... as well as the
clear battering it received from impacts.
But is Mars a dead world?
An infrared telescope on Hawaii's Mauna Kea
volcano was trained on Mars over several years.
Astronomers used a spectrometer to split the
light into its individual wavelengths... to
identify chemical fingerprints in Mars' atmosphere.
They found the signature of methane gas, in
amounts that change from place to place.
Methane wouldn't last very long under Mars'
conditions... there must be some source that
replenishes it.
That source could be biological... perhaps
microbes alive and well somewhere underground.
Or the methane could be coming from volcanic
activity... or maybe chemical reactions in
Mars' crust.
So far, neither the satellites flying over
Mars... nor the robots on the surface... have
turned up anything close to clear proof of
living organisms.
It may take the searching eyes, flexible minds,
and nimble fingers of human explorers to find
that buried treasure, if it exists.
In the meantime, we are finding that even
if Mars appears dead, it's certainly not dull.
Though Mars has nowhere near the dynamism
of Earth, with our oceans... turbulent atmosphere...
volcanism... and shifting continents... Mars
does do some fascinating things:
If you take an atmosphere, however tenuous...
add heat from the sun so it blows over the
dry plains... you get the renowned dust devils
of Mars.
With no trees to hold down the soil ... landslides
are common.
The Mars Reconnaissance Orbiter has returned
images showing that Mars is actively remaking
its surface...
Not in canals built by the Martian engineers
of Percival Lowell's imagination...
But in picturesque sand dunes shaped by the
wind...
And in intricate landscapes molded by a gradually
changing climate...
Whether or not we ever find life-forms on
Mars, we can still marvel at the beauty of
its landforms... subtly sculpted over eons
of time...
on a world that never was.
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